Perhaps the scariest thing we have heard so far about Resident Evil: The Mercenaries is that it operates on a permasave system; you cannot reset data on the cartridge. Whilst nothing can be done about this directly, it is possible to wipe the cartridge clean using a product called the NDS Adaptor Plus.

The device was designed to back up save data from the Nintendo DS and 3DS to PC, but it can also be used to erase saved games. Its a bit of pain but, if you want a clean Mercenaries cart, it is a solution.

Thanks to Siliconera for the info.